1. Field of the Invention
The present invention relates to liquid crystal display devices such as active matrix type devices including a plurality of thin film transistors. The invention more particularly relates to a liquid crystal display device which can prevent variations in video images such as a drop in the brightness caused by rounding of the waveform of video signals.
2. Description of the Related Art
In conventional active matrix type liquid crystal display devices, there is a source driver to drive a thin film transistor provided on a one-pixel-basis. FIG. 1 is a circuit diagram of the configuration of a conventional liquid crystal display device. In the conventional liquid crystal display device, a thin film transistor is provided for each of the pixels arranged in a matrix. A plurality of thin film transistors T11, . . . , Tm1, . . . , Tn1, . . . forming the first column have their drains connected in common to a drain line D1. Similarly, a plurality of thin film transistors T12, . . . , Tm2, . . . , Tn2, . . . forming the second column have their drains connected in common to a drain line D2. Thus, a plurality of thin film transistors T1a, . . . , Tma, . . . , Tna, . . . forming the a-th column have their drains connected in common to a drain line Da.
The drain lines are connected with output buffers B1, B2, B3, B4, . . . provided at a source driver 11.
A plurality of thin film transistors T11, T12, T13, T14, . . . forming the first row have their gates connected in common to a gate line G1. Similarly, a plurality of thin film transistors Tm1, Tm2, Tm3, Tm4, . . . forming the m-th row have their gates connected in common to a gate line Gm, and a plurality of thin film transistors Tn1, Tn2, Tn3, Tn4, . . . forming the n-th row have their gates connected in common to a gate line Gn. Thus, a plurality of thin film transistors Tb1, Tb2, Tb3, Tb4, . . . forming the b-th row have their gates connected in common to a gate line Gb.
In the conventional liquid crystal display device having the above-described configuration, a video signal is supplied from the output buffers B1, B2, . . . to the drain lines D1, D2, . . . , respectively. The gate lines G1, . . . , Gm, . . . , Gn, . . . are supplied with a control signal from a vertical driver (not shown), and each thin film transistor turns on/off in response to the control signal. When the thin film transistor is turned on, the video signal supplied to the corresponding drain line is applied to the liquid crystal for the pixel, so that a video image based on the video signal is displayed on the display.
In the conventional display device described above, however, there are resistance and capacitance parasitic in the drain line, and its time constant increases from the input end on the output buffer side to the terminal end on the opposite side. As a result, the video signal is rounded in the waveform.
More specifically, as shown in FIG. 1, when one vide signal is output from the output buffer B1 to the drain line D1, the thin film transistor T11 connected to the gate line G1 in the first row is provided with a normal square signal, but the thin film transistor Tm1 connected to the gate line Gm in the m-th row is provided with a signal having a rounded waveform. Furthermore, the thin film transistor Tn1 connected to the gate line Gn in the n-th row provided farther from the output buffer B1 is provided with a signal having a more rounded waveform. When the distance from the output buffer B1 exceeds a prescribed value, the wave height at the time of falling is lower than a prescribed level.
A pixel stores a signal voltage at the time of falling of the signal, and therefore if the value decreases, the luminance changes, which causes variations in video images. If, for example, an image in white is to be displayed on the entire display screen, the brightness decreases as the distance from the output buffer increases.
Therefore, in order to prevent such variations in video images depending on the distance from the source driver, a liquid crystal display device which outputs a video signal from both sides of the drain line has been suggested (Japanese Patent Laid-Open Publication No. 10-274762).
The conventional liquid crystal display device disclosed in this publication could reduce variations in video images compared to the devices before then, but the disadvantage associated with the waveform rounding is not solved. In the central part of the drain line, there exist video image variations. In addition, this technique requires two drivers in some cases, and therefore should not be considered sufficient in terms of reduction in the area and the cost.